Abnormality detecting system for oxygen sensor and abnormality detecting method

ABSTRACT

An abnormality detecting system and method detect an abnormality in an oxygen sensor provided on a downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine. An upstream side sensor is provided on an upstream side of the catalyst, and produces an output corresponding to an exhaust air-fuel ratio. A theoretical oxygen storage capacity of the catalyst is obtained, for example, by determining an amount of oxygen in the exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the upstream side sensor produces a lean output. An abnormality in the oxygen sensor is detected to exist when the theoretical oxygen storage capacity has exceeded a maximum oxygen storage amount of the catalyst.

INCORPORATION BY REFERENCE

[0001] The disclosure of Japanese Patent Application No. 2002-175274 filed on Jun. 17, 2002, including the specification, drawings and abstract is incorporated herein by reference in its entirety

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The invention relates to an abnormality detecting system for an oxygen sensor and an abnormality detecting method. More particularly, the invention relates to an abnormality detecting system and method which detects an abnormality in an oxygen sensor that is provided on the downstream side of a catalyst for purifying exhaust gas.

[0004] 2. Description of Related Art

[0005] As disclosed in Japanese Patent Laid-Open Publication No. 06-273371, a system which includes an oxygen sensor on a downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine is known. This system controls an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be rich when the oxygen sensor produces a lean output, and then detects an abnormality in the sensor when the output from the oxygen sensor is not inverted to a rich output until a predetermined time elapses.

[0006] When the system controls the air-fuel ratio of the air-fuel mixture to be rich, exhaust gas containing unburned components such as HC and CO, that is, oxygen-deficient exhaust gas, flows into the catalyst. When oxygen has been stored in the catalyst, the oxygen is released, and HC and CO are thereby oxidized in the catalyst. Consequently, purified exhaust gas which does not contain HC and CO is released downstream of the catalyst.

[0007] When the air-fuel ratio of the air-fuel mixture is maintained at a rich value, the oxygen in the catalyst is completely consumed with time, and the exhaust gas containing HC and CO, that is the oxygen-deficient exhaust gas, flows downstream of the catalyst. A normally functioning oxygen sensor inverts its output to a rich output when contacting such exhaust gas. The above-mentioned conventional system detects an abnormality in the sensor when the output from the oxygen sensor is not inverted even when time which is usually sufficient to completely consume the oxygen in the catalyst has elapsed since the air-fuel ratio of the air-fuel mixture is controlled to be rich. According to such a method, it is possible to accurately detect an abnormality in the oxygen sensor.

[0008] The time which is necessary to completely consume the oxygen in the catalyst varies according to a flow amount of the exhaust gas flowing into the catalyst. Also, the flow amount of the exhaust gas flowing into the catalyst varies according to an operation state of the internal combustion engine. Accordingly, in the conventional system, the time which is sufficient to completely consume the oxygen in the catalyst after the air-fuel ratio is controlled to be rich varies according to the operation state of the internal combustion engine and the like.

[0009] In order to accurately detect an abnormality in the oxygen sensor at all times, it is necessary to set the predetermined time, that is, the time in which to wait for the inversion of the output from the sensor after the air-fuel ratio is controlled to be rich, based on the assumption that the flow amount of the exhaust gas is at a minimum. Therefore, according to the conventional system, when a large amount of exhaust gas is generated, it may take an unnecessarily long time until an abnormality in the oxygen sensor is detected after the air-fuel ratio of the air-fuel mixture is controlled to be rich.

SUMMARY OF THE INVENTION

[0010] The invention is made in order to address the problem described above. Accordingly, it is one object of the invention to provide an abnormality detecting system which can detect an abnormality in an oxygen sensor provided on the downstream side of a catalyst in the shortest time regardless of an operation state of an internal combustion engine.

[0011] According to one aspect of the invention, an abnormality detecting system is provided which detects an abnormality in an oxygen sensor provided on the downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine. This abnormality detecting system includes an upstream side sensor which is provided on an upstream of the catalyst and which produces an output corresponding to an air-fuel ratio of the exhaust gas (hereinafter, referred to as an “exhaust air-fuel ratio”), and a controller. The controller determines an amount of oxygen in exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the upstream side sensor produces a lean output, so as to obtain a theoretical oxygen storage capacity of the catalyst. The controller detects that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded a maximum oxygen storage amount of the catalyst.

[0012] According to a further aspect of the invention, an abnormality detecting method is provided. This method detects an abnormality in the oxygen sensor that is provided on the downstream side of the catalyst for purifying the exhaust gas released from the internal combustion engine. The abnormality detecting method includes the steps of: (I) detecting an exhaust air-fuel ratio upstream of the catalyst; (2) obtaining a theoretical oxygen storage capacity of the catalyst by determining the amount of oxygen in the exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the exhaust air-fuel ratio upstream of the catalyst is detected to be lean; and (3) detecting that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded the maximum oxygen storage amount of the catalyst.

[0013] According to this abnormality detecting system and method, it is possible to obtain the theoretical oxygen storage capacity of the catalyst by the following method. The abnormality detecting system determines the amount of the oxygen in the exhaust gas during the period in which the oxygen sensor produces the rich output and the upstream side sensor produces the lean output. Namely, the abnormality detecting system determines the amount of the oxygen flowing into the catalyst (the amount of the oxygen stored in the catalyst) during the period in which the exhaust gas containing oxygen flows into the catalyst and the purified exhaust gas flows out of the catalyst.

[0014] Then, it is possible to detect an abnormality in the oxygen sensor when the theoretical oxygen storage capacity thus obtained has exceeded the maximum oxygen storage amount of the catalyst. Accordingly, it is possible to accurately detect an abnormality in the oxygen sensor in a short time regardless of the operation state of the internal combustion engine.

[0015] According to a further aspect of the invention, an abnormality detecting system is provided which detects an abnormality in an oxygen sensor that is provided on the downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine. The abnormality detecting system includes an upstream side sensor which is provided on the upstream side of the catalyst and which produces an output corresponding to an exhaust air-fuel ratio, and a controller. The controller determines an amount of oxygen deficiency in exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the upstream side sensor produces a lean output, so as to obtain a theoretical oxygen storage capacity of the catalyst. The controller detects that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded the maximum oxygen storage amount of the catalyst.

[0016] According to a further aspect of the invention, an abnormality detecting method is provided. This method detects an abnormality in the oxygen sensor that is provided on the downstream side of the catalyst for purifying the exhaust gas released from the internal combustion engine. The abnormality detecting method includes the steps of: (1) detecting an exhaust air-fuel ratio upstream of the catalyst; (2) obtaining a theoretical oxygen storage capacity of the catalyst by determining an amount of oxygen deficiency in the exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the exhaust air-fuel ratio upstream of the catalyst is detected to be lean; and (3) detecting that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded the maximum oxygen storage amount of the catalyst.

[0017] According to this abnormality detecting system and method, it is possible to obtain the theoretical oxygen storage capacity of the catalyst by the following method. The abnormality detecting system determines the amount of the oxygen deficiency in the exhaust gas during the period in which the oxygen sensor produces the rich output and the upstream side sensor produces the lean output. Namely, the abnormality detecting system determines the amount of the oxygen deficiency in the exhaust gas flowing into the catalyst (the amount of the oxygen released from the catalyst) during the period in which the oxygen-deficient exhaust gas flows into the catalyst and the purified exhaust gas flows out of the catalyst.

[0018] Then, it is possible to detect an abnormality in the oxygen sensor when the theoretical oxygen storage capacity thus obtained has exceeded the maximum oxygen storage amount of the catalyst. Accordingly, it is possible to accurately detect an abnormality in the oxygen sensor in a short time regardless of the operation state of the internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above mentioned and other objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of exemplary embodiments of the invention, when considered in connection with the accompanying drawings, in which:

[0020]FIG. 1 is a diagram describing a configuration of a first embodiment of the invention;

[0021]FIG. 2 is a diagram describing a configuration of an oxygen sensor included in a system according to the first embodiment;

[0022]FIG. 3 is a timing chart describing operation of the system according to the first embodiment when active control is performed in a state in which the oxygen sensor functions properly;

[0023]FIG. 4 is a timing chart describing operation of the system according to the first embodiment when the active control is performed in a state in which there is an abnormality in the oxygen sensor;

[0024]FIG. 5 is a flowchart of a control routine performed by the system according to the first embodiment; and

[0025]FIG. 6 is a diagram describing an output characteristic of the oxygen sensor included in the system according to the first embodiment

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] In the following description and the accompanying drawings, the invention will be described in more detail in terms of exemplary embodiments. The same reference numerals will be assigned to the components common to the drawings, and overlapping description will be omitted.

[0027]FIG. 1 is a diagram describing a configuration of a first embodiment of the invention. A system shown in FIG. 1 includes an internal combustion engine 10. An intake passage 12 and an exhaust passage 14 communicate with the internal combustion engine 10.

[0028] An air flow meter 18 is provided on a downstream side of an air filter 16, in the intake passage 12. The air flow meter 18 is a sensor for detecting an amount Ga of the intake air (hereinafter, referred to as an “intake air amount Ga”) flowing through the intake passage 12. A throttle valve 20 is provided on a downstream side of the air flow meter 18. In addition, a fuel injection valve 22 for injecting fuel into an intake port of the internal combustion engine 10 is provided in the intake passage 12.

[0029] A catalyst 24 communicates with the exhaust passage 14. The catalyst 24 can store a certain amount of oxygen. When NOx is contained in the exhaust gas, the catalyst 24 purifies the exhaust gas by reducing the NOx, and can store the oxygen released in the reduction process. When an unburned component such as HC and/or CO is contained in the exhaust gas, the catalyst 24 can purify the exhaust gas by oxidizing the unburned component while releasing the stored oxygen. Here, it is to be understood that “storage” used herein means retention of a substance (solid, liquid, gas molecules) in the form of at least one of adsorption, adhesion, absorption, trapping, occlusion, and others.

[0030] An air-fuel ratio sensor 26 is provided on an upstream side of the catalyst 24, and an oxygen sensor 28 is provided on a downstream side of the catalyst 24, in the exhaust passage 14. The air-fuel ratio sensor 26 is a sensor for producing an output corresponding to an exhaust air-fuel ratio. According to the air-fuel ratio sensor 26, it is possible to detect the air-fuel ratio of the exhaust gas which is just released from the internal combustion engine 10, that is an air-fuel ratio of the exhaust gas before being purified by the catalyst 24.

[0031] The oxygen sensor 28 is a sensor which greatly changes the output therefrom depending on the presence or absence of oxygen in the exhaust gas. Therefore, according to the oxygen sensor 28, it is possible to accurately detect the presence or absence of oxygen in the exhaust gas flowing downstream of the catalyst 24.

[0032]FIG. 2 is a diagram describing the configuration of the oxygen sensor 28. As shown in FIG. 2, the oxygen sensor 28 includes a heater 30 and an element layer 32. The element layer 32 is configured so as to surround the heater 30. An electrode 34, which is formed so as to surround a tip portion of the heater 30, is embedded in the element layer 32. Also, an air space 36, which is supplied with air, is formed inside the element layer 32. In addition, a measurement gas chamber 40 surrounded by a cover 38 is formed outside the element layer 32.

[0033] The electrode 34 generates electromotive forces depending on the presence or absence of oxygen on a surface thereof facing the air chamber 36 and on a surface thereof facing the measurement gas chamber 40. The oxygen sensor 28 outputs a difference between these electromotive forces as its output. When the exhaust gas containing oxygen is introduced to the measurement gas chamber 40, the electrode 34 generates an electromotive force corresponding to the presence of oxygen on both the surface thereof facing the air chamber 36 and the surface thereof facing the measurement gas chamber 40. In this case, the oxygen sensor produces an output substantially equal to 0V.

[0034] Meanwhile, when the exhaust gas which does not contain oxygen is introduced into the measurement gas chamber 40, the electrode 34 generates an electromotive force corresponding to the presence of oxygen on the surface thereof facing the air chamber 36, and generates an electromotive force corresponding to the absence of oxygen on the surface thereof facing the measurement gas chamber 40. In this case, the oxygen sensor produces an output of approximately 1V.

[0035] As described so far, the oxygen sensor 28 greatly changes its output depending on the presence or absence of oxygen in the exhaust gas flowing downstream of the catalyst 24. Therefore, according to the output from the oxygen sensor 28, it is possible to accurately detect whether the exhaust gas flowing downstream of the catalyst 24 contains oxygen.

[0036] As shown in FIG. 1, the system according to the embodiment includes an Electronic Control Unit (hereinafter, referred to as an ECU) 50. The ECU 50 is a unit for controlling operation of the system according to the embodiment. Outputs from the various sensors are supplied to the ECU 50, and the fuel injection valve 22 is connected to the ECU 50. The ECU 50 can control the fuel injection amount based on the outputs from the sensors.

[0037] Next, operation of the system according to the embodiment will be described. In the embodiment, the ECU 50 performs stoichiometric control during normal operation. In the stoichiometric control, the fuel injection amount is controlled such that the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine 10 is maintained in the vicinity of the stoichiometric air-fuel ratio. More particularly, the fuel injection amount is controlled such that the exhaust air-fuel ratio detected by the air-fuel ratio sensor 26 alternately becomes rich and lean within a small range with respect to the stoichiometric air-fuel ratio.

[0038] When the exhaust air-fuel ratio is lean, exhaust gas containing NOx flows into the catalyst 24. In this case, the catalyst 24 purifies the exhaust gas by reducing NOx, and stores the oxygen thus generated. Accordingly, when the exhaust air-fuel ratio is lean, the oxygen storage amount of the catalyst 24 tends to increase. When the exhaust air-fuel ratio is inverted to the rich air-fuel ratio, exhaust gas containing HC and CO flows into the catalyst 24. In this case, the catalyst 24 oxidizes the HC and CO while releasing the stored oxygen. Consequently, purified exhaust gas flows downstream of the catalyst.

[0039] Thus, while the ECU 50 performs the stoichiometric control, the catalyst 24 alternately stores and releases oxygen. Consequently, the purified exhaust gas continuously flows downstream of the catalyst 24. Therefore, according to the system in the embodiment, it is possible to realize a good exhaust emission characteristic during normal operation.

[0040] The ECU performs active control when a predetermined condition is satisfied. In the active control, the target air-fuel ratio of the air-fuel mixture is inverted between a predetermined rich target value (for example, 14.1) and a predetermined lean target value (for example, 15.1) each time the output from the oxygen sensor 28 is inverted.

[0041]FIG. 3 is a timing chart describing the operation of the system when the active control is performed in the case where the oxygen sensor 28 functions properly. More particularly, (A) of FIG. 3 shows a change (a waveform (1)) in the target air-fuel ratio and a change (a waveform (2)) in the exhaust air-fuel ratio A/F detected by the air-fuel ratio sensor 26 during the active control. (B) of FIG. 3 shows a change (a waveform (3)) in the output from the oxygen sensor 28.

[0042] The timing chart in FIG. 3 shows an example of a case in which the stoichiometric control is performed until time t1, and then the active control is started. In this example, at time t1, the output from the oxygen sensor 28 is inverted from the rich output to the lean output, and the target air-fuel ratio is changed from the stoichiometric air-fuel ratio to the rich target value at the start time of the active control.

[0043] After the target air-fuel ratio is changed to the rich target value, the ECU 50 gradually increases the fuel injection amount until the exhaust air-fuel ratio A/F detected by the air-fuel ratio sensor 26 reaches the rich target value. Consequently, the exhaust air-fuel ratio A/F becomes a value in the vicinity of the rich target value after a certain time lag from time t1.

[0044] While the exhaust air-fuel ratio A/F is maintained at a rich value, the rich exhaust gas, that is, the oxygen-deficient exhaust gas containing HC and CO flows into the catalyst 24. When such oxygen-deficient exhaust gas flows into the catalyst 24 retaining the stored oxygen, the oxygen in the catalyst 24 is released, and HC and CO are oxidized (purified) In this case, the purified exhaust gas containing oxygen flows downstream of the catalyst 24. Therefore, while the catalyst 24 retains the stored oxygen, the output from the oxygen sensor 28 provided on the downstream side of the catalyst 24 is maintained at a lean value.

[0045] In FIG. 3, time t2 shows the time when the stored oxygen in the catalyst 24 has been completely released. When the stored oxygen in the catalyst 24 has been completely released, the catalyst 24 becomes unable to release oxygen into the exhaust gas. Therefore, in such a case, the oxygen-deficient exhaust gas containing HC and CO then starts flowing downstream of the catalyst 24. Consequently, at time t2, the output from the oxygen sensor 28 is inverted from the lean output to the rich output.

[0046] The ECU 50 can determine that the catalyst 24 has completely released the stored oxygen when the output from the oxygen sensor 28 is inverted. In the active control, when the output from the oxygen sensor 28 is inverted in such a manner, the target air-fuel ratio is inverted to the lean target value at this time (t2).

[0047] After the target air-fuel ratio is changed to the lean air-fuel ratio at time t2, the ECU 50 gradually decreases the fuel injection amount until the exhaust air-fuel ratio A/F detected by the air-fuel ratio sensor 26 reaches the lean target value. Consequently, the exhaust air-fuel ratio A/F becomes a value in the vicinity of the lean target value after a time lag from time t2.

[0048] While the exhaust air-fuel ratio A/F is maintained at a lean value, the lean exhaust gas, that is, the oxygen-excess exhaust gas containing NOx flows into the catalyst 24. When the exhaust gas containing NOx flows into the catalyst 24 which has a reserve capacity for storing oxygen, the catalyst 24 reduces NOx while storing the oxygen thus generated. In this case, the purified exhaust gas which does not contain oxygen flows downstream of the catalyst 24. Therefore, while the catalyst 24 has the reserve capacity for storing oxygen, the output from the oxygen sensor 28 provided on a downstream side thereof is maintained at a rich value.

[0049] In FIG. 3, time t3 shows the time when the catalyst 24 has stored oxygen to its fullest extent. After the catalyst 24 has stored oxygen to its fullest extent, the oxygen in the exhaust gas starts flowing downstream of the catalyst 24. Consequently, the output from the oxygen sensor 28 is inverted from the rich output to the lean output at time t3. The ECU 50 can determine that the catalyst 24 has stored oxygen to its fullest extent when the output from the oxygen sensor 28 is inverted. In the active control, when the output from the oxygen sensor 28 is inverted in such a manner, the target air-fuel ratio is inverted to the rich target value at this time (t3). During the active control, the processing is alternately performed in which the target air-fuel ratio is inverted between the rich target value and the lean target value each time the output from the oxygen sensor 28 is inverted (e.g., times t4, t5)

[0050] As described so far, during the active control, a state in which the catalyst 24 has stored oxygen to its fullest extent and a state in which the catalyst 24 has completely released the stored oxygen are alternately realized according to the inversion of the target air-fuel ratio. The hatched areas in (A) of FIG. 3 show a period from when the catalyst 24 is empty until when the catalyst 24 has stored oxygen to its fullest extent, or a period from when the catalyst has stored oxygen to its full extent until when the catalyst 24 becomes empty, in the process in which the above-mentioned two states are inverted. Therefore, it is possible to calculate an oxygen storage capacity Cmax of the catalyst 24, by accumulating the amount of the oxygen flowing into the catalyst 24 (at the storage time), or by accumulating the amount of the oxygen deficiency in the exhaust gas flowing into the catalyst 24 (at the release time), during the periods. A specific method for calculating Cmax will be described later.

[0051] As described so far, according to the system in the embodiment, it is possible to obtain the oxygen storage capacity Cmax of the catalyst 24 by performing the active control when the oxygen sensor 28 functions properly. The oxygen storage capacity Cmax tends to decrease with deterioration of the catalyst 24. In the embodiment, the ECU 50 can detect a degree of the deterioration based on Cmax calculated by the above-mentioned method.

[0052]FIG. 4 is a timing chart describing the operation of the system when the active control is performed in the case in which there is an abnormality in the oxygen sensor 28. Wave forms (1) to (3) shown in FIG. 4 show the change (the wave form (1)) in the target air-fuel ratio, the change (the wave form (2)) in the exhaust air-fuel ratio A/F detected by the air fuel sensor 26, and the change (the wave form (3)) in the output from the oxygen sensor 28, respectively, as shown in FIG. 3. The timing chart in FIG. 4 shows an example of a case in which the stoichiometric control is performed until time t1, and then the active control is started. In this example, at time t1, the target air-fuel ratio is changed from the stoichiometric air-fuel ratio to the rich target value at the start time of the active control.

[0053] After the target air-fuel ratio is changed to the rich target value, the ECU 50 gradually increases the fuel injection amount until the exhaust air-fuel ratio A/F reaches the rich target value. Consequently, the exhaust air-fuel ratio A/F becomes a value in the vicinity of the rich target value after a time lag from time t1. While the exhaust air-fuel ratio A/F is maintained at a rich value, the oxygen-deficient exhaust gas containing HC and CO flows into the catalyst 24. While stored oxygen remains in the catalyst 24, HC and CO are oxidized (purified), and the purified exhaust gas containing oxygen flows downstream of the catalyst 24. After the oxygen in the catalyst 24 has been completely consumed, oxygen-deficient exhaust gas containing HC and CO starts flowing downstream of the catalyst 24 (time t2).

[0054] As described above with reference to FIG. 3, when the oxygen sensor 28 functions properly, the output from the oxygen sensor 28 is inverted at time t2. Meanwhile, when there is an abnormality in the oxygen sensor 28, even after the oxygen-deficient exhaust gas starts flowing downstream of the catalyst 24, the output from the oxygen sensor 28 is not inverted, and the lean output is maintained, as shown in (B) of FIG. 4.

[0055] During the active control, when the output from the oxygen sensor 28 is inverted, a command for inverting the target air-fuel ratio is issued. Accordingly, when the output from the oxygen sensor 28 is not inverted, the target air-fuel ratio is maintained at a rich target value, as shown in (A) of FIG. 4. A hatched area in (A) of FIG. 4 shows a period in which the ECU 50 attempts to calculate the oxygen storage capacity Cmax of the catalyst 24 by accumulating the amount of the oxygen deficiency in the exhaust gas flowing into the catalyst 24. In this case, since the accumulation is performed for an unnecessarily long time, the theoretical oxygen storage capacity Cmax becomes an unnecessarily large value. Accordingly, in the embodiment, when the calculated value of the oxygen storage capacity Cmax becomes unnecessarily large due to the active control, the ECU 50 detects an abnormality in the oxygen sensor 28 and stops the active control at this time.

[0056]FIG. 5 shows a flow chart of a control routine performed by the ECU 50 in the embodiment so as to realize the above-mentioned function. In the routine shown in FIG. 5, it is determined whether the detection of an abnormality in the oxygen sensor 28 has been completed (step S100).

[0057] When it is determined that the detection of an abnormality has been completed, the present processing cycle ends promptly. Meanwhile, when it is determined that the detection of an abnormality has not been completed, it is determined whether a condition for performing the active control has been satisfied (step S102).

[0058] As a result, when it is determined that the condition for performing the active control has not been satisfied, the present processing cycle ends promptly. Meanwhile, when it is determined that the condition has been satisfied, it is determined whether the output from the oxygen sensor has been inverted from the previous processing cycle to the present processing cycle (step S104).

[0059] When it is determined that the output from the oxygen sensor 28 has been inverted, it can be determined that the oxygen sensor 28 functions properly. Accordingly, in the case where such a determination is made, the present processing cycle ends after it is determined that the oxygen sensor functions properly (step S106). When it is determined in step S106 that the oxygen sensor 28 functions properly, it is determined that the detection of an abnormality is completed at this time. Accordingly, when the routine is restarted after step S106 is performed, it is determined in step S100 whether the detection of an abnormality has been completed.

[0060] When it is determined in step S104 that the output from the oxygen sensor 28 has not been inverted, it is determined whether the output from the oxygen sensor 28 is a rich output (step S108). Namely, taking the timing chart in FIG. 3 as an example, it is determined whether a state shown between time t2 and time t3, or a state shown between time t4 and time t5 has been established.

[0061] When it is determined that the output from the oxygen sensor 28 is a rich output, it is further determined whether the output from the air-fuel ratio sensor 26 is lean (step S110). Namely, taking the period between time t2 and time t3, or the period between time t4 and time t5 as an example, it is determined whether a state shown by a hatched period in FIG. 3 has been further established during the period.

[0062] As a result, when it is determined that the condition in step S110 has not been satisfied, it can be determined that although the target air-fuel ratio is inverted to the lean target value, the air-fuel ratio of the exhaust gas flowing into the catalyst 24 has not become lean yet. In this case, the value of the storage time Cmax is reset to be an initial value (step S112).

[0063] Note that the “storage time Cmax” is the oxygen storage capacity of the catalyst 24, which is calculated by accumulating the amount of oxygen flowing into the catalyst 24 in the process in which the catalyst 24 stores oxygen.

[0064] Meanwhile, when it is determined in step S110 that the exhaust air-fuel ratio A/F is lean, it can be determined that the target air-fuel ratio has been inverted to the lean target value, and further the air-fuel ratio of the exhaust gas flowing into the catalyst 24 has become lean. In this case, the storage time Cmax is calculated according to the following equation (step S114).

Cmax=CmaxO+0.23×ΔA/F×Fuel amount

[0065] Note that CmaxO is an initial value (0) of Cmax, or the latest calculated value of Cmax, and 0.23 is an oxygen ratio in the air. Also, ΔA/F is the value obtained by subtracting the stoichiometric air-fuel proportion from the exhaust air-fuel ratio A/F detected by the air-fuel ratio sensor 26. Further, the fuel amount is the amount of the fuel supplied to the internal combustion engine 10 during the repeating (clock) period (for example, 65 msec) of the routine. In this case, the ECU 50 detects the fuel amount based on the fuel injection amount calculated in another routine.

[0066] In the equation, “ΔA/F×Fuel amount” corresponds to the amount of the unburned air which flows into the catalyst 24 during the repeating period of the routine. The value obtained by multiplying the above-mentioned value by 0.23 corresponds to the amount of the unburned oxygen. Therefore, according to the equation, it is possible to obtain the accumulated value of the amount of the oxygen which flows (is stored) into the catalyst 24 during the repeating period of the routine each time step S114 is performed.

[0067] In the routine shown in FIG. 5, it is determined whether the storage time Cmax calculated in the processing in step S114 is larger than the reference value a (step S116). The reference value a is the initial value of the oxygen storage capacity of the catalyst 24, that is, the value corresponding to the oxygen storage capacity of the catalyst 24 at the time of factory shipment. Although the oxygen storage capacity of the catalyst 24 may decrease with time, it does not increase. Accordingly, when Cmax which exceeds a is calculated, it can be determined that the output from the oxygen sensor 28 has not been inverted for an unnecessarily long time, that is, there is an abnormality in the oxygen sensor 28.

[0068] When it is determined in step S116 that Cmax>α has not been established, it cannot be determined whether there is an abnormality in the oxygen sensor 28. In this case, the present processing cycle ends while determination of the presence or absence of an abnormality is suspended.

[0069] Meanwhile, when it is determined in step S116 that Cmax>α has been established, it is determined that there is a rich abnormality in the oxygen sensor 28 (step S118). When it is determined in the process in step S118 that there is the rich abnormality in the oxygen sensor 28, it is determined that the detection of an abnormality has been completed at this time. Accordingly, when the routine is restarted after step S118 is performed, it is determined in step S100 that the detection of an abnormality has been completed. The “rich abnormality” in the oxygen sensor 28 is an abnormality in which the output from the oxygen sensor 28 remains on the rich side, and the lean output cannot be produced.

[0070] After it is determined that there is the rich abnormality in the oxygen sensor 28, the command for canceling the active control is issued (step S120), after which the present processing cycle ends.

[0071] When it is determined in step S108 in the routine in FIG. 5 that the output from the oxygen sensor 28 is not the rich output, it can be determined that the output is the lean output. Taking the timing chart in FIG. 3 as an example, it can be determined that the state shown between time t1 and time t2, or the state shown between time t3 and time t4 has been established.

[0072] When the above-mentioned determination is made, it is determined whether the output from the air-fuel ratio sensor 26 is rich (step S122). Namely, taking the period between time t3 and time t4 in FIG. 3 as an example, it is determined whether a state of the hatched period in FIG. 3 has been further established during the period. As a result, when it is determined that a condition in step S122 has not been satisfied, it can be determined that although the target air-fuel ratio has been inverted to the rich target value, the air-fuel ratio of the exhaust gas flowing into the catalyst 24 has not become rich yet; In this case, the value of the release time Cmax is reset to be the initial value (step S124). The “release time Cmax” is the oxygen storage capacity of the catalyst 24 which is calculated by accumulating the amount of the oxygen deficiency in the exhaust gas flowing into the catalyst 24 in the process in which the catalyst 24 releases oxygen.

[0073] Meanwhile, when it is determined in step S122 that the exhaust air-fuel ratio A/F is rich, it can be determined that the target air-fuel ratio has been inverted to the rich target value, and further the air-fuel ratio of the exhaust gas flowing into the catalyst 24 has become rich. The release time Cmax is calculated according to the same equation (Cmax=Cmax0+0.23×ΔA/F×Fuel amount) as in the case of step S114 (step S126). However, the “ΔA/F×Fuel amount” calculated in step S126 is the amount of the oxygen which is necessary to burn the unburned components (HC, CO) in the exhaust gas, that is, the amount of the oxygen deficiency in the exhaust gas. According to the processing in step S126, it is possible to obtain the accumulated value of the oxygen released from the catalyst 24 during the repeating period of the routine.

[0074] In the routine shown in FIG. 5, it is determined whether the release time Cmax calculated in the processing in step S126 is larger than the reference value a (step S128). As mentioned above, the reference value a is the initial value of the oxygen storage capacity of the catalyst 24.

[0075] When it is determined in step S128 that Cmax>α has not been satisfied, it cannot be determined whether there is an abnormality in the oxygen sensor 28. In this case, the present processing cycle ends while detection of an abnormality is suspended.

[0076] Meanwhile, when it is determined in step S128 that Cmax>α has been satisfied, it is determined that there is a lean abnormality in the oxygen sensor 28 (step S130). When it is determined that there is the lean abnormality in the oxygen sensor 28 in the processing in step S130, it is determined that the detection of an abnormality has been completed at this time. Accordingly, when the routine is restarted after step S130 is performed, it is determined in step S100 that the detection of an abnormality has been completed. The “lean abnormality” in the oxygen sensor 28 is an abnormality in which the output from the oxygen sensor 28 remains on the lean side and the rich output cannot be produced.

[0077] When it is determined that there is the lean abnormality in the oxygen sensor 28, the present processing cycle ends after the processing in step S120 is performed so as to cancel the active control.

[0078] As described so far, according to the routine shown in FIG. 5, the storage time Cmax can be calculated in the state in which the catalyst 24 needs to store oxygen during the active control (the state in which the output from the oxygen sensor 28 is rich, and the output from the air-fuel ratio sensor 26 is lean). Then, the theoretical Cmax has exceeded the initial value of the oxygen storage capacity, it is possible to promptly detect an abnormality in the oxygen sensor 28, and further to cancel the active control.

[0079] According to the routine shown in FIG. 5, the release time Cmax can be calculated in a state in which the catalyst 24 needs to release oxygen during the active control (a state in which the output from the oxygen sensor 28 is lean, and the output from the air-fuel ratio sensor 26 is rich). When the theoretical Cmax has exceeded the initial value of the oxygen storage capacity, it is possible to promptly detect an abnormality in the oxygen sensor 28, and further to cancel the active control.

[0080] According to the above-mentioned method, it is possible to detect an abnormality in the oxygen sensor 28 according to the operation state of the internal combustion engine 10, that is, according to the flow amount of the exhaust gas, in the shortest time at all operating conditions of the internal combustion engine. Then, it is possible to minimize the amount of the unpurified exhaust gas released to the air by promptly canceling the active control after an abnormality in the oxygen sensor 28 is detected. Therefore, according to the system in the embodiment, when there is an abnormality in the oxygen sensor 28, it is possible to accurately detect the abnormality in a short time, and to adequately suppress deterioration of the emission characteristic.

[0081] An abnormality due to a crack in an element in addition to an abnormality due to a short-circuit or a disconnection may be caused in the oxygen sensor 28 which is employed in the embodiment. In the case of the abnormality due to a short-circuit or a disconnection, since the output from the sensor remains on one of the output sides, it is possible to detect the presence or absence of an abnormality by checking whether a change is caused in the output from the sensor. Meanwhile, it is not possible to detect the presence or absence of an abnormality due to a crack in the element by such a method.

[0082] The abnormality in the oxygen sensor 28 due to a crack in the element is an abnormality in which a crack is caused in the element layer 32 shown in FIG. 2. When this abnormality occurs, it becomes possible that the air introduced into the air space 36 intrudes into the measurement gas chamber 40 through a crack. FIG. 6 is a diagram showing a normal output characteristic (solid line) and an output characteristic (dashed line) when a crack is caused in the element, of the oxygen sensor 28. As shown in this diagram, the oxygen sensor 28 in which a crack exists in the element produces a low output when the air-fuel ratio of the exhaust gas is rich or lean, and produces a high output when the air-fuel ratio of the exhaust gas is in the vicinity of the stoichiometric air-fuel ratio.

[0083] When the output characteristic of the oxygen sensor 28 is as shown by a dashed line in FIG. 6, it is not possible to detect the presence or absence of the abnormality depending on whether there is a change in the output from the sensor. Since the oxygen sensor 28 is provided on the downstream side of the catalyst 24, it cannot be determined based on the output from the air-fuel ratio sensor 26 whether the exhaust gas flowing around the catalyst 24 is rich or lean in actuality. Accordingly, it is not possible to detect the presence or absence of the abnormality due to a crack in the element simply by comparing the output from the air-fuel ratio sensor 26 with the output from the oxygen sensor 28.

[0084] Meanwhile, according to the method employed in the embodiment, in the case where the rich exhaust gas flows around the oxygen sensor 28, when the oxygen sensor 28 continues to produce an output which is in the vicinity of 0V, an abnormality in the sensor is promptly detected. Accordingly, the system in the embodiment can detect the presence or absence of an abnormality due to a crack in the element of the oxygen sensor 28 in a short time.

[0085] In the first embodiment, the active control in which the target air-fuel ratio is repeatedly inverted is performed so as to detect the presence or absence of an abnormality in the oxygen sensor 28. However, the invention is not limited to this. Namely, in order to detect an abnormality in the oxygen sensor 28, it is not necessary to invert the target air-fuel ratio between the rich target value and the lean target value. The target air-fuel ratio may be set to either the rich target value or the lean target value until the presence or absence of an abnormality in the oxygen sensor 28 is determined.

[0086] In the first embodiment, the air-fuel ratio is forcibly controlled to be rich or lean by controlling the target air-fuel ratio to the rich target value or the lean target value so as to determine the presence or absence of an abnormality in the oxygen sensor 28. However, the invention is not limited to this. Namely, the presence or absence of an abnormality in the oxygen sensor 28 may be determined using the period in which the air-fuel ratio is necessarily on the rich or the lean side, such as the period during fuel cutting.

[0087] In the first embodiment, the amount of the oxygen flowing into the catalyst 24 and the amount of the oxygen deficiency in the exhaust gas flowing into the catalyst 24 are calculated based on the difference ΔA/F between the exhaust air-fuel ratio A/F detected by the air-fuel ratio sensor 26 and the stoichiometric air-fuel ratio, and the amount of the fuel supplied to the internal combustion engine 10. However, the invention is not limited to this. Namely, the amount of the oxygen and the amount of the oxygen deficiency may be calculated based on the intake air amount detected by the air flow meter 18 and the fuel supply amount (the fuel amount), without using ΔA/F.

[0088] When such a calculation method is employed, it is unnecessary to measure the air-fuel ratio A/F upstream of the catalyst 24, unlike the first embodiment. Accordingly, when this calculation method is employed, the sensor provided on the upstream side of the catalyst 24 may be an oxygen sensor instead of an air-fuel ratio sensor.

[0089] In the first embodiment, it is determined that the oxygen sensor 28 is in a normal state when an inversion of the output from the oxygen sensor 28 is determined once in a process of performing the routine in FIG. 5 (refer to steps S104, S106). However, the invention is not limited to this. Namely, determination whether the oxygen sensor 28 is in a normal state may not be made until both an inversion from the rich output to the lean output, and an inversion from the lean output to the rich output are determined.

[0090] The controller (e.g., the ECU 50) of the illustrated exemplary embodiments is implemented as a programmed general purpose computer. It will be appreciated by those skilled in the art that the controller can be implemented using a single special purpose integrated circuit (e.g., ASIC) having a main or central processor section for overall, system-level control, and separate sections dedicated to performing various different specific computations, functions and other processes under control of the central processor section. The controller can be a plurality of separate dedicated or programmable integrated or other electronic circuits or devices (e.g., hard wired electronic or logic circuits such as discrete element circuits, or programmable logic devices such as PLDs, PLAs, PALs or the like). The controller can be implemented using a suitably programmed general purpose computer, e.g., a microprocessor, microcontroller or other processor device (CPU or MPU), either alone or in conjunction with one or more peripheral (e.g., integrated circuit) data and signal processing devices. In general, any device or assembly of devices on which a finite state machine capable of implementing the procedures described herein can be used as the controller. A distributed processing architecture can be used for maximum data/signal processing capability and speed.

[0091] While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention. 

What is claimed is:
 1. An abnormality detecting system for detecting an abnormality in an oxygen sensor which is provided on a downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine, comprising: an upstream side sensor which is provided on an upstream side of the catalyst and which produces an output corresponding to an exhaust air-fuel ratio; and a controller which obtains a theoretical oxygen storage capacity of the catalyst by determining an amount of oxygen in exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the upstream side sensor produces a lean output, wherein the controller detects that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded a maximum oxygen storage amount of the catalyst.
 2. The abnormality detecting system according to claim 1, wherein the upstream side sensor is an air-fuel ratio sensor for detecting an exhaust air-fuel ratio, and the controller further obtains a difference ΔA/F between an air-fuel ratio detected by the upstream side sensor and a stoichiometric air-fuel ratio; detects a fuel supply amount to the internal combustion engine; and calculates an amount of oxygen in the exhaust gas based on the difference ΔA/F and the fuel supply amount.
 3. The abnormality detecting system according to claim 2, wherein the controller forcibly controls a target air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be lean while the oxygen sensor produces the rich output, so as to calculate the theoretical oxygen storage capacity.
 4. The abnormality detecting system according to claim 1, wherein the controller forcibly controls a target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to be lean while the oxygen sensor produces the rich output, so as to calculate the theoretical oxygen storage capacity.
 5. The abnormality detecting system according to claim 3, wherein the controller further forcibly controls the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to be rich while the oxygen sensor produces the lean output, and inverts the target air-fuel ratio between rich and lean each time the output from the oxygen sensor is inverted by forcibly controlling the target air-fuel ratio to be rich and lean alternately.
 6. The abnormality detecting system according to claim 3, wherein the controller prohibits forcing setting of the target air-fuel ratio when an abnormality in the oxygen sensor is detected.
 7. An abnormality detecting system for detecting an abnormality in an oxygen sensor provided on a downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine, comprising: an upstream side sensor which is provided on an upstream side of the catalyst and which produces an output corresponding to an exhaust air-fuel ratio; and a controller which obtains a theoretical oxygen storage capacity of the catalyst by determining an amount of oxygen deficiency in exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a lean output and the upstream side sensor produces a rich output, wherein the controller detects that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded a maximum oxygen storage amount of the catalyst.
 8. The abnormality detecting system according to claim 7, wherein the upstream side sensor is an air-fuel ratio sensor for detecting an exhaust air-fuel ratio, and the controller further obtains a difference ΔA/F between an air-fuel ratio detected by the upstream side sensor and a stoichiometric air-fuel ratio; detects a fuel supply amount to the internal combustion engine; and calculates an amount of oxygen deficiency in the exhaust gas based on the difference ΔA/F and the fuel supply amount.
 9. The abnormality detecting system according to claim 8, wherein the controller forcibly controls a target air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be rich while the oxygen sensor produces the lean output, so as to calculate the theoretical oxygen storage capacity.
 10. The abnormality detecting system according to claim 7, wherein the controller forcibly controls a target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to be rich while the oxygen sensor produces the lean output, so as to calculate the theoretical oxygen storage capacity.
 11. A method for detecting an abnormality in an oxygen sensor provided on a downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine, comprising the steps of: detecting an exhaust air-fuel ratio upstream of the catalyst; obtaining a theoretical oxygen storage capacity of the catalyst by determining an amount of oxygen in the exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a rich output and the exhaust air-fuel ratio upstream of the catalyst is detected to be lean; and detecting that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded a maximum oxygen storage amount of the catalyst.
 12. The abnormality detecting method according to claim 11, further comprising the steps of: obtaining a difference ΔA/F between an air-fuel ratio detected upstream of the catalyst and a stoichiometric air-fuel ratio; detecting a fuel supply amount to the internal combustion engine; and calculating an amount of oxygen in the exhaust gas based on the difference ΔA/F and the fuel supply amount.
 13. The abnormality detecting method according to claim 12, further comprising the step of: forcibly controlling a target air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be lean while the oxygen sensor produces the rich output.
 14. The abnormality detecting method according to claim 11, further comprising the step of: forcibly controlling a target air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be lean while the oxygen sensor produces the rich output.
 15. The abnormality detecting method according to claim 14, further comprising the steps of: forcibly controlling the target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to be rich while the oxygen sensor produces the lean output; and inverting the target air-fuel ratio between rich and lean by alternately switching the target air-fuel ratio between lean and rich each time the output from the oxygen sensor is inverted.
 16. The abnormality detecting method according to claim 15, further comprising the step of: prohibiting forcing setting of the target air-fuel ratio when an abnormality in the oxygen sensor is detected.
 17. A method for detecting an abnormality in an oxygen sensor provided on a downstream side of a catalyst for purifying exhaust gas released from an internal combustion engine, comprising the steps of: detecting an exhaust air-fuel ratio upstream of the catalyst; obtaining a theoretical oxygen storage capacity of the catalyst by determining an amount of oxygen deficiency in the exhaust gas flowing into the catalyst during a period in which the oxygen sensor produces a lean output and the exhaust air-fuel ratio upstream of the catalyst is detected to be rich output; and detecting that there is an abnormality in the oxygen sensor when the theoretical oxygen storage capacity has exceeded a maximum oxygen storage amount of the catalyst.
 18. The abnormality detecting method according to claim 17, further comprising the steps of: obtaining a difference ΔA/F between an air-fuel ratio detected upstream of the catalyst and a stoichiometric air-fuel ratio; detecting a fuel supply amount to the internal combustion engine; and calculating an amount of oxygen deficiency in the exhaust gas based on the difference A A/F and the fuel supply amount.
 19. The abnormality detecting method according to claim 18, further comprising the step of: forcibly controlling a target air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine to be rich while the oxygen sensor produces the lean output.
 20. The abnormality detecting method according to claim 17, further comprising the step of: forcibly controlling a target air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to be rich while the oxygen sensor produces the lean output. 